The present invention relates to burning sulfur-containing fuels in a fluidized bed boiler and, more particularly, to an improved process of burning sulfur-containing coal in a fluidized bed of sulfur oxide sorbent material wherein utilization of the sulfur sorbent is increased.
The ever growing public awareness of the environment has led to the enactment of legislation at the national, state and local levels directed at preserving our environment for future generations. Particular attention has been given to sulfur dioxide emissions resulting in the promulgation of federal regulations severely restricting emissions of sulfur dioxide in the flue gas generated during the combustion of fossil fuel.
One obvious method of complying with such regulations is to burn only fossil fuels which contain little or no sulfur, such as natural gas and light oils. However, the scarcity of the known domestic reserves of low sulfur oil and natural gas coupled with the high cost of foreign supplies of such fuels precludes the burning of these clean fuels as a viable solution to our air pollution problem.
Domestic supplies of coal are, on the other hand, abundant. Estimates have been given that domestic supplies of coal could satisfy our nation's energy need for the next two to three hundred years. Unfortunately coal is not a clean burning fuel like natural gas or low sulfur oil. Coals found in the United States typically contain sulfur in amounts ranging from about 100 to 1300 nanograms per Joule of heating value. Since any sulfur contained in the coal would, when combusted in the same manner as a clean fuel, be readily converted to sulfur dioxide and emitted to the atmosphere, much attention has been directed to developing methods of burning sulfur-containing fuels such as coal while at the same time preventing pollution of the atmosphere with sulfur dioxide. As a result, interest has been rekindled in the burning of coal in a fluidized bed boiler and, in particular, to the burning of coal in a fluidized bed of sulfur oxide sorbent.
The great potential for minimizing emissions of sulfur dioxide to the atmosphere when burning sulfur-containing fuels such as coal in a fluidized bed of sulfur oxide sorbent has been recognized for some time. For example, British Pat. No. 824,883, issued in 1959, discloses burning a sulfur-containing solid fuel in a fluidized bed of sulfur sorbent such as limestone or dolomite.
In the typical present day fluidized bed boiler, particulate coals having a topsize ranging from 3.0 to 6.5 millimeters are typically fed to and combusted within a fluidized bed of similarsized limestone particles at a relatively low temperature of 760 C. to 925 C. under oxidizing conditions. During combustion within the bed, a major portion of the sulfur dioxide generated reacts with the limestone within the bed thereby forming calcium sulfate which is retained within the bed. Typically, calcium utilization at these conditions is about 20 to 35 percent. Calcium utilization is defined as the overall fractional conversion of available calcium sorbent in the limestone to calcium sulfate via reaction with sulfur dioxide generated during the combustion of a sulfur-containing fuel within the bed. Limestone must be continually fed to the bed at a rate sufficient to maintain the calcium to sulfur mole ratio, defined as the ratio of moles calcium in the limestone feed to moles sulfur in the coal feed, from two-to-one to four-to-one in order to maintain an acceptable sulfur dioxide retention within the bed.
A number of approaches have been suggested for improving calcium utilization in the limestone bed. One approach has been to use extremely fine limestone having a particle size passing a 325 mesh screen, i.e., having a maximum particle size of about 40 microns, as the sulfur absorbing compound within the fluidized bed. However, this approach poses serious problems relating to material handling and, in particular, to increased dust loading when the flue gas is leaving the fluidized bed. In fact, one air pollution problem is substituted for another. That is, a sulfur dioxide emission problem is eliminated; but a particulate emission problem is created. Because of their small size, such fine limestone particles are readily blown upward out of the bed by the fluidizing air which is maintained at a velocity high enough to fluidize the coarser coal particles. As a result of this elutriation of the fine limestone particles from the bed, elaborate and very expensive dust collection equipment must be provided to remove the fine limestone particles from the flue gas prior to venting this flue gas to the atmosphere.
Another approach has been to provide a system for removing the spent sulfur oxide sorbent from the bed and treating it to regenerate its sulfur oxide absorbing capability. One such regeneration system is illustrated in U.S. Pat. No. 3,717,700 wherein the bed drain material, which includes ash, unburned carbon and spent limestone particles, is heated in a slightly oxidizing atmosphere in a second fluidized bed with a carbonaceous fuel to a temperature in the range of 925 C. to 1150 C. to drive off the sulfur retained by the sorbent as SO.sub.2 thereby regenerating the sulfur oxide sorption capability of the sorbent. Other known schemes for regenerating the spent sorbent also require heating the spent sorbent in either a reducing or an oxidizing atmosphere. Such regeneration processes all share one major drawback--the energy consumption required to heat the spent sorbent to the high temperatures required to drive off the absorbed sulfur from the spent sorbent. Additionally, the absorbed sulfur is typically driven off as SO.sub.2 or H.sub.2 S gas which must be removed from the flue gas of the regeneration vessel by a process such as wet scrubbing before venting the flue gas to the atmosphere.
Other approaches which have been suggested include thermally pretreating the limestone before feeding it to the bed to increase its sulfur oxide sorption activity or adding other chemicals to the limestone bed to catalyze the sulfur oxide-calcium reaction. These approaches, however, have proven impractical economically and technologically.